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Homework Help: Battery problem

  1. Feb 23, 2008 #1
    1. The problem statement, all variables and given/known data
    An Ordinary flashlight uses two D-cell 1.5V batteries connected in series. The bulb draws 450mA when turned on. A)Calculate the resistance of the bulb and the power dissipated. B) By what factor would the power increase if 4 D-cell batteries in series were used with the same bulb? (neglect heating effects)

    2. Relevant equations
    P=IV=I^2R=V^2/R V=IR

    3. The attempt at a solution
    I understand part A, and I got 1.4W for the power. For part B though, I keep getting that it's 2 times greater by doing P=IV where (.45A)(4 x 1.5) =2.7. but I know the answer is supposed to be 4 times. I'm pretty sure the current stays constant and that you add voltages, so I'm confused why it would be 4 and not 2.
  2. jcsd
  3. Feb 23, 2008 #2


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    You should post these questions either in introductory physics or in the engineering homework help subforum.

    I get P = 1.35 W for part a), and 5.4 W for part b)

    Doubling the voltage doubles the current, which quadruples the power.

    EDIT: Your mistake is that you used the original current in your calculation for part b), rather than using the new current once having calculated it.
    Last edited: Feb 23, 2008
  4. Feb 23, 2008 #3
    Thank you. I did post this in the introductory forum about 3 hours ago but no one was able to respond so I thought maybe I needed to put it here, figuring someone would be able to answer it quickly--which fortunately was the case. Sorry to take your time.
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